I'm currently designing a keep for my players to explore. It will be a ruined keep, taken over by an evil cult. But to start off the design, I am looking at how it would have been designed in its prime. Here are some rooms I've thought of when creating the layout.

Military function

• Armory
• Smithy
• Fletcher
• Training room
• Lookout posts
• Infirmary/Hospital wing
• Gatehouse
• Barbican
• Place of Arms
• Commander quarters
• Guard bunks

Every day operation

• Great hall (mess hall)
• Steward’s Office
• Clerks/Tax assessors/Scribes Office
• Kitchen
• Distillery
• Scullery
• Larder/Storeroom/Cellar/Ice House
• Servant quarters
• Bathroom
• Well/Cistern
• Dovecote
• Stables
• Kennels
• Courtyard
• Gardens

Political function

• Hall of Justice
• Dungeon/Prison/Oubliette/Tower
• Throne Room
• Vault/Treasury

Noble quarters

• Drawing Room
• Bower
• Rooms for the Lord/Lady
• Formal sitting room
• Informal sitting room
• Private study
• Personal library
• Private dining room

Research

• Library
• Alchemy lab
• Solarium
• Wizards quarters

Religious

• Temple/Church
• Clergy quarters

Can you think of some more?

Editing to add the suggestions from the comments.